This invention is directed to a network for automatically and quickly cutting off the beam current of a television receiver's CRT (cathode ray tube) when the receiver is turned off.
When a television receiver is turned off, its vertical and horizontal deflection voltages collapse rapidly. However, the cathodes of the receiver's CRT continue to emit electrons for a short time because of their elevated temperatures. Consequently, if the CRT's electron guns are not biased off, the residual emission of the cathodes produces a visible spot near the center of the CRT. Prolonged and repeated generation of such a spot may permanently burn the CRT where the spot is developed.
To avoid burning the CRT, it is common to include a network for automatically reverse biasing each electron gun when the receiver is turned off, thereby to cut off any electron beam resulting from residual cathode emission. A typical network which provides such beam cut-off is shown in FIG. 1, to which reference is now made.
The illustrated network includes a pair of resistors R.sub.1 and R.sub.2 and a diode D.sub.1 which are serially connected between a 250 volt supply and ground. The junction between the resistors R.sub.1 and R.sub.2 is coupled to the grid G.sub.1 of an electron gun to provide a bias of about plus 25 volts on the grid during normal receiver operation. A capacitor C.sub.1 is coupled between ground and the grid to provide an A.C. bypass.
To reverse bias the grid when the receiver is turned off, a large capacitor C.sub.2 is coupled between the diode D.sub.1 and the 250 volt supply. This capacitor charges with the polarity indicated to a value of about 250 volts in normal receiver operation. When the receiver is turned off, the 250 volt supply collapses, but the diode D.sub.1 prevents the capacitor C.sub.2 from discharging rapidly. Hence, the charge stored on the capacitor C.sub.2 reverse biases the G.sub.1 grid to turn its electron gun off quickly.
Although the illustrated network and others like it perform satisfactorily, they are impractical to integrate, primarily because of the need for two capacitors. This drawback renders such conventional networks undesirable for use in modern receivers.